Amines are a very important family of compounds in chemistry and biology. They are widely used in the production of pharmaceuticals, fine chemicals, agrochemicals, polymers, dyestuffs, pigments, emulsifiers and plasticizing agents (1). Among the amines, the terminal primary amines (e.g. RNH2 wherein R is an organic radical) are the most useful, but their selective synthesis is challenging, due to their high reactivity.
The conversion of alcohols to amines by conventional methods typically involves two to three steps, each step generally requiring isolation and purification, making the process cumbersome for even small-scale syntheses (2). Fewer classical methods are known for the stepwise, one-pot conversion of alcohols to primary amines, although such methods are not environmentally benign and are inappropriate for large scale production (3-6). Existing methods for the preparation of primary amines generally utilize stoichiometric amounts of toxic reagents and lead to poor selectivity and very low atom-economy (7-9). An attractive method for the preparation of secondary and tertiary linear amines by hydroaminomethylation of internal olefins was reported (10). Amines are also prepared by the reduction of amides, generally under harsh conditions to result in a mixture of products (11). Iridium and rhodium catalyzed preparation of amines from aldehydes was also reported (12). Although high hydrogen pressure is required for the reductive amination of aldehydes, and alcohols are formed as by-products, this method demonstrated the first homogeneous catalytic reductive amination process with ammonia. Lewis acid catalyzed reductive amination methods for the synthesis of amines are also known (13, 14). Recently, synthesis of arylamines was achieved by palladium-catalyzed arylation of ammonia in dioxane (15). Primary amines can be alkylated by alcohols to obtain secondary amines (16). Iridium-catalyzed multialkylation of ammonium salts with alcohols was reported for the synthesis of secondary and tertiary amines, but selective synthesis of primary amines remains as a tantalizing task (17).
Among the methods utilized for commercial production of amines (1, 18), by far, the largest and most utilized are based on the reaction of alcohols with ammonia. However, the solid acid catalyzed reaction of alcohols with ammonia requires very high temperatures (300-500° C.) and forms mixtures of primary, secondary and tertiary amines, and also large amounts of alkene as a result of dehydration. The metal-oxide catalyzed reaction of alcohols and amines at high temperature and pressure also results in mixtures of amines and has to be conducted under hydrogen pressure for catalyst stability. In addition, this reaction forms alkanes as a result of CO extrusion. (18)
Catalytic coupling of ammonia with organic substrates for the direct preparation of aryl and alkyl primary amines are considered to be two of the ten greatest challenges in catalysis (19). Atom economical methods to activate alcohols (replacing the Mitsunobu protocol) for the direct nucleophilic substitution and “N”-centered chemistry that precludes azides and hydrazine are among the most required processes in pharmaceutical industries (20). Selective catalytic synthesis of primary amines is a paradoxical challenge as the primary amines are more nucleophilic than ammonia and compete with it in reaction with electrophiles such as alkyl halides or aldehydes, producing secondary amines (21), which can also react, leading to the formation of mixtures of products.
Thus, selective, catalytic synthesis of primary amines directly from alcohols and ammonia with elimination of water, under relatively mild conditions, without producing waste, is highly desirable economically and environmentally. However, such a facile process is unknown.
Pincer complexes can have outstanding catalytic properties (22, 23). The applicants of the present invention previously reported the dehydrogenation of alcohols catalyzed by PNP- and PNN-Ru(II) hydride complexes (24). Whereas secondary alcohols lead to ketones (25, 26), primary alcohols are efficiently converted into esters and dihydrogen (25-26). A dearomatized PNN pincer complex was particularly efficient (28); it catalyzed this process in high yields under neutral conditions, in the absence of acceptors or promoters. US patent application publication no. US 2009/0112005, to the applicants of the present invention, describes methods for preparing amides, by reacting a primary amine and a primary alcohol in the presence of ruthenium catalysts, to generate the amide compound and molecular hydrogen.
Given the widespread importance of amines in biochemical and chemical systems, an efficient synthesis that avoids the shortcomings of prior art processes is highly desirable.